Twin-wire former

ABSTRACT

The twin-wire former serves for the production of a web of paper or board. Two wire belts (endless wire loops 11 and 12) form a twin-wire zone with each other. Within the twin-wire zone, the one wire belt (12) travels over rigid ledges (28&#39;, 28) which are arranged spaced apart from each other on a water-removal box (18). Within the twin-wire zone, furthermore, the other wire belt (11) travels over several ledges (27) which lie opposite the rigid ledges (28), are supported by means of resilient elements (springs 24, pneumatic pressure cushions, or the like), and can be pressed with a selectable force against the other wire belt. Within one of the wire loops (for instance 11), bridging at least two of the ledges (27) present there, a closed wire support surface (9) is provided. A secondary headbox (10&#39;) can be arranged in front of the closed wire support surface (9).

The present invention relates to a twin-wire former for the productionof a fiber web, in particular a web of paper or board, from a fibersuspension, and particularly to a twin-wire former having ledges abovethe forming wires of the twin-wire former for aiding in directing thewater away from the wires. In such a twin-wire former, there are twopaper machine wire belts or wires which together form a twin-wire zone,and the fiber suspension travels between the belts. Each of the wirebelts travels over a respective plurality of ledges which are arrangedon the outsides of the belts. The ledges in each plurality thereof arearranged at a distance apart from each other. One plurality of ledgesabove one wire are rigid ledges. The other plurality of ledges above theother wire are resiliently supported to be pressed against the otherwire belt. The following publications are indicated as prior art:

1. GB 2174120 A

2. EP 0371786 A2

3. WO 91/02842

4. DE-OS 40 05 420 which is equivalent to U.S. Pat. No. 5,045,153

5. EP 0405154 A1

Documents 1 and 2 disclose different twin-wire formers in each of whichthe top wire travels along the (substantially flat) lower side of adewatering box. According to EP '786 this dewatering box has rigidledges on its bottom. Directly below said rigid ledges the bottom wiretravels over ledges which can be applied against it resiliently. Inseveral embodiments of GB '120 the ledges are close together so thatwater cannot discharge downward through the bottom wire in this region.The same is true of other embodiments of GB '120 in which a flexibleplate is provided instead of ledges. In further embodiments of GB '120,smaller or larger spaces are present between resiliently appliableledges, which spaces can receive smaller or larger amounts of water anddischarge them laterally to the outside. This is true also of thetwin-wire former in accordance with EP '786. In all of these cases, theentire flat lower surface of the dewatering box which is arranged in thetop wire is covered by the resiliently appliable ledges present in thebottom wire or by the said flexible plate, with the exception of at mostthe narrow zones of the intermediate spaces. This has the result thatthe discharge of the water downward is prevented to a greater or lesserextent in the entire region of the flat bottom side of the dewateringbox.

Therefore, one of the disadvantages of all of these known arrangementsis that the dewatering takes place exclusively (or practicallyexclusively) in the upward direction in the region of the resilientlyappliable ledges (or of the flexible plate). Therefore, the quality ofthe fiber webs produced leaves something to be desired, in particularwith regard to the "formation" or "cloudiness". There is also theproblem that the said intermediate spaces become clogged with thepassage of time, so that the formation is not uniform over the width ofthe web.

Therefore, a construction was adopted in the prior art which onlyrelatively few ledges which could be pressed resiliently against the onewire are provided. Here, large spaces which can receive large amounts ofwater are present between the ledges. Furthermore, openings are providedso that these amounts of water can discharge downward over the shortestpossible path. Twin-wire formers of this type are described inPublications 3 and 4. In general, the following is true of the twin-wireformers in accordance with Publications 1-4: Due to the resilientlysupported ledges which are arranged opposite the rigid ledges, thefollowing can be achieved: For instance, upon an increase in the amountof suspension flowing in between the two wire belts, the resilientlysupported ledges can move away somewhat. In this way, the danger (whichexists when rigidly supported ledges alone are used) of a damming upoccurring in the fiber suspension in front of the ledges is eliminated.Such a damming up could destroy the fiber layers formed up to that timeon the two wire belts. In other words: In the known twin-wire formers inaccordance with Publications 1-4, a dewatering pressure which has oncebeen set remains constant due to the resiliently supported ledges evenupon a change in the amount of suspension fed or upon a change in thedewatering behavior of the fiber suspension. An automatic adaptation ofthe width of the gap between the fixed and resilient ledges thereforetakes places when one of the said changes occurs. The known arrangementstherefore permit the production of webs having a very large range ofbasis weights, namely from relatively thin paper webs to relativelythick board webs.

With the twin-wire formers known from Publication 3 or 4, fiber webs ofrelatively good "formation" (i.e. with uniform distribution of fiber --or, in other words, with good "cloudiness") can be formed. In thisconnection, however, in recent days the requirements have increasedconsiderably so that further improvements are desirable.

The object of the present invention is, therefore, to develop atwin-wire former in such a manner that the quality of the fiber webproduced is further improved, particularly with respect to its formation(cloudiness).

This object is achieved by the features below. In accordance with afirst aspect of the invention, a wire support surface is provided in theinitial region of that part of the twin-wire zone in which thestationary and resilient ledges are opposite each other-- and/ordirectly in front of this part of the twin-wire zone-- over whichsupport surface one of the two wire belts travels. This wire supportsurface is preferably completely water-impermeable; however, it may alsobe of limited water permeability. In any event, it is seen to it in theregion of this wire support surface that the removal of water takesplace "temporarily" exclusively (or almost exclusively) through theopposite wire belt ("temporarily" means only in a relatively smallinitial region of the said part of the twin-wire zone). The normal waterremoval on both sides is therefore intentionally shifted a distancefurther in the direction of travel of the web. By this measure, aconsiderable improvement in the formation is surprisingly obtained, asshown by experiments.

This favorable result can be obtained independently of the direction oftravel of the wire belts through the twin-wire zone, and therefore withhorizontal, inclined, or vertical direction of travel of the wire. Inthe case of predominantly horizontal direction of travel of the wirebelts through the twin-wire zone, the resiliently supported ledges arein general associated with the bottom wire. In that case, it isadvantageous to associate the said wire support surface also with thebottom wire. However, it is also possible to arrange the wire supportsurface within the loop of the top wire; this may be advantageous if thetwin-wire former has a single-wire pre-water-removal zone. In general,by the selection of the arrangement of the wire support surface ineither one or the other wire loop, the distribution of the fines andfillers within the thickness of the fiber web to be produced can becontrolled.

In accordance with a second aspect of the invention, a wire supportsurface is provided in each of the two wire loops rather than in onlyone. In this case, the arrangement is effected in such a manner that thetwo wire support surfaces overlap each other in whole or in part. Thefollowing explanations relate to twin-wire formers having only a singlewire support surface; they apply, however, by analogy also when two wiresupport surfaces are present opposite each other.

The following is again emphasized:

The essentially water-impermeable wire support surface provided inaccordance with the invention which temporarily prevents the dischargeof water is to be present only at the start of the said part of thetwin-wire zone.

In other words, the invention is based on the discovery that, differingfrom all the previous designs, the removal of water through one of thetwo wires must be temporarily braked or prevented only in the initialregion of the zone in which rigid and resiliently supported ledges lieopposite each other. In this way, it is possible to produce fiber websof the highest quality (particularly with regard to the "formation" )and to do so-- as previously-- within a very large range of basisweights, from relatively thin paper webs up to relatively thick boardwebs. An indispensable requirement for this is that an essential part ofthe formation of the web take place in that part of the twin-wire zonein which the said resiliently supported ledges cooperate with theopposite rigidly supported ledges, in which connection-- as alreadymentioned-- the substantially water-impermeable wire support surface ofthe invention must be provided in the initial part of this zone.

Publication 5 (EP '154) describes a twin-wire former of a differenttype. In that case, the twin-wire zone is formed by a curvedwater-removal box which lies in the loop of the bottom wire and has onits top initially a curved shoe followed by several stationarilysupported ledges arranged at a distance apart along the curved path oftravel of the wire. Above this water-removal box, there is present inthe loop of the top wire another water-removal box which, however,contacts the upper wire only by a single ledge which is arranged behindthe lower water-removal box. To be sure, the discharge of water indownward direction is temporarily interrupted by said shoe. Cooperationof this shoe with rigid and resilient ledges which lie opposite eachother-- as explained above-- is, however, neither disclosed norsuggested in EP '154.

The part of the twin-wire zone in which rigid and resiliently supportedledges lie opposite each other and in which at least a part of thesubstantially water-impermeable wire support surface of the invention islocated will be referred to below as the "sandwich zone". The length ofthe wire support surface is between 10 and 60% of the length of the"sandwich zone". The length of the wire support surface will be adaptedto the operating conditions prevailing in the individual case (inparticular, with respect to the speed of the machine and the basisweight of the web to be produced). The position of the wire supportsurface may differ; it can, for instance, lie in part in front of and inpart within the "sandwich zone". As an alternative to this, it can bearranged completely within the "sandwich zone". In a preferredconstruction, the position of the wire support surface is variablewithin the above said limits.

In order to eliminate the danger of damming up occurring in the fibersuspension (as described above) in front of the wire support surface(seen in the direction of travel), it is advantageous to press the wiresupport surface against the bottom wire by means of resilient elements(spring, pressure cushions or the like). The pressing force can befreely selected within certain limits (as in the case of the resilientledges), for instance by changing the spring force or the cushionpressure.

If the twin-wire former in accordance with the invention has (in knownmanner) a predominantly horizontally extending single-wirepre-water-removal zone, a secondary headbox can be provided shortlybefore the start of the twin-wire zone. By means of it, a second layercan be delivered onto the pre-dewatered first fiber layer. As a rule,the two layers have different properties, for instance different colors.In this case, an additional advantage is obtained by means of the wiresupport surface of the invention, which in this case supports the bottomwire; namely, the result is obtained that the second suspension layer isnot directly dewatered after the feeding thereof through the first layerwhich has already been pre-dewatered. Rather, the second layer ofsuspension is dewatered initially exclusively (or almost exclusively) inupward direction. In this way, it is avoided that a component of thesecond suspension layer, for instance the coloring substance, penetratesrapidly into the first layer. In other words, the result is obtainedthat certain different properties of the layers, for instance differentcolors, remain unchanged up to the completion of the web of paper orboard.

Further concepts of the invention are concerned with the problem offurther developing a plate which forms the wire support surface. Asalready mentioned above, this plate is pressed from below against thebottom wire by resilient members, preferably pneumatic pressure cushionswhose pressure is variable. During operation, the plate should befastened securely on the resilient elements with respect to thedirection of travel of the wire. Nevertheless, it should be capable ofbeing easily pushed in and out transverse to the direction of travel ofthe wire, for instance, in order to change its position in the directionof travel of the wire or simply in order to replace it by another one.Another problem consists in developing the plate in such a manner thatall regions thereof rest with relatively little application of forcesnugly against the bottom of the bottom wire. This will be trueprimarily of several zones of the plate which follow one another in thedirection of travel of the wire and extend transverse to the directionof travel of the wire. Solutions of these additional problems are givenherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will be described below withreference to the drawings.

Each of FIGS. 1 to 8 shows diagrammatically one of the variousembodiments, in part in side view and in part in longitudinal section.

FIGS. 9, 10 and 11 show structural details in different embodiments.

FIG. 12 is a diagrammatic cross section through the initial region of atwin-wire zone having a closed wire support surface in the form of afoil.

In FIG. 1, two wire belts 11 and 12 (with the fiber suspension 1 whichis in part still liquid between them) travel in the direction indicatedby the arrow R between a lower water-removal box 17 and an upperwater-removal box 18. The lower water-removal box is provided on itsfront end (as seen in the direction of travel of the wire) with a rigidledge 8 which, however, can also be omitted. It is followed at avariable distance by a closed, and therefore water-impermeable, plate 9Awhich forms a wire support surface 9 for the bottom wire belt 11. Theplate is supported on a rigid water-permeable plate 26 via ledges 27Aand compression springs 24A (the spring force of which is adjustable) orvia pneumatic pressure cushions. Following plate 9A in the direction oftravel of the web there are several ledges 27 (of, for instance,approximately rectangular cross section) which are pressed resilientlyfrom below against the bottom wire 11. For this purpose they aresupported, for instance via compression springs (or via pneumaticpressure cushions), on the rigid water-permeable plate 26. It is obviousthat the force of the compression springs 24 (or the pressure prevailingin the pressure cushions) can be adjusted individually at eachindividual ledge 27. A preferred construction of the ledges 27 and oftheir vertical guidance is described in DE 40 19 884 which is equivalentto U.S. Pat. No. 5,078,835. The following alternative is not shown: Theledges 27 rest on a flexible plate which is supported by a plurality ofpneumatic pressure cushions. In accordance with a further alternative,the plate 9A could be provided with relatively fine vertical holes orslits which permit a "braked"discharge of water in downward direction.

The upper water-removal box 18, on which a guide roll 14 for the topwire 12 is supported, can be suspended both on its front end and on itsrear end as indicated schematically by the double-ended arrows P and P',on approximately vertically displaceable support elements, not shown.Thus, the position of the guide roll 14 and of the box 18 can beadjusted, if necessary, even during operation. On the bottom of the box18 there is a row of, for instance, at least eight ledges 28, 2' having,for instance, a parallelogram-shaped cross section, which rest againstthe top of the top wire 12 and are firmly attached to the box 18. Abovethe ledges 28, 28' a front vacuum chamber 21 and a rear vacuum chamber22 are provided in the water-removal box 18. In front of the upperwater-removal box 18, the top wire 12 travels over the said guide roll14. It is therefore assumed in FIG. 1 that the bottom wire 11 forms asubstantially horizontal single-wire pre-water-removal path between aheadbox (not shown) and the place where it comes together with the topwire (see FIG. 2). The fiber suspension which has been pre-dewatered butstill contains in part liquid fiber suspension is shown in exaggeratedthickness in FIG. 1. It can be seen, however, that box 18 and guide roll14 are so adjusted that the top wire comes into contact with the top ofthe fiber suspension between guide roll 14 and the first ledge 28',namely at the place K. The feed side edge (or "front edge") of the plate9A is also present approximately there. Its discharge-side edge (or"rear edge") lies approximately below the third ledge 28 of the box 18.The zone in which the upper ledges 8 lie opposite the lower ledges 27and a part of the plate A is the so-called "sandwich zone" S.

In accordance with FIG. 1, the following is provided as example: In theregion of the upper water-removal box 18, the number of rigid ledges 28is greater (preferably about twice as great) as the number of lower,resiliently supported ledges 27. On the upper water-removal box, thedistances between two adjacent ledges is approximately two to four timesthe thickness of the ledges. In the case of the lower ledges, thesedistances are substantially greater. Within the length of the upper box18, each of the lower ledges 27 lies opposite a gap between two upperledges 28. Every two or three upper ledges 28 lie opposite a gap betweentwo lower ledges 27. (Differing from FIG. 1, the upper and lower ledgescan also be at approximately the same distances from each other; seeFIGS. 2-5).

The dewatering boxes 17 and 18 are followed by, for instance, a curvedsuction box 23 arranged in the lower wire 20 or by a similar curvedsuction box 23' in the form of an extension of the box 18, arranged inthe top wire 12.

Upon the operation of the twin-wire former, an intensive two-sidedremoval of water (downward and upward) takes place in the region wherethe lower and upper ledges 27, 28 are opposite each other, each of theledges 27, 28 producing, by a slight (scarcely visible) deflection ofthe corresponding wire belt 11 or 12 in the still liquid part of thefiber material, a pressure pulse which effects a more uniformdistribution of the fiber (for instance, breaks up flocks). This actionis intensified by the fact that at the start of the twin-wire zone theremoval of water in downward direction is temporarily interrupted or atleast braked by the plate 9A so that here water removal takes placeexclusively, or almost exclusively, in upward direction. Accordingly,the zone in which the lower ledges 27 produce the said pressure pulsesin the still liquid fiber material is shifted in the direction of travelof the web. The extent of this shift can be varied in the manner thatthe position of the plate 9A is changed in the direction of travel ofthe web or opposite thereto; see, for instance, the position designated9'. Or else a plate of a different length L is inserted. However, as arule, at least the first upper ledge 28' should be opposite the plate9A. The length L of the plate 9A (measured in the direction of travel ofthe wire) is in FIG. 1 about 50% of the length of the sandwich zone S.In general, the length L of plate 9A will be in the range of 10% to 60%of the length of the sandwich zone S.

FIG. 1 also shows diagrammatically other possible variants: As analternative or in addition to the plate 9A which supports the bottomwire 11, a plate 90, the bottom of which (wire support surface 9a)contacts the top wire 12, can be provided in the loop of the top wire12. The plate 90 is preferably arranged at the place of the first (forinstance, two or three) ledges 28' and 28, for instance fastened oncorrespondingly shortened ledges. If the lower plate 9A is also present,the two plates 9A and 90 overlap, at least in part. The position and/orlength of the plate 90 is variable in the same way as the plate 9A.

In accordance with FIG. 2, the bottom wire 11 travels toward a headbox10 over a breast roll 13 and then over water removal elements 16a, 16band 16c. The last of these water-removal elements is developed as acurved suction box 16c; from here the bottom wire 11 travels with aslight inclination downward over a shoe 9B and over lower ledges 27resiliently supported on a box 17. The surface of the shoe 9B forms awater-impermeable wire support surface 9 for the bottom wire 11. Theshoe 9B is supported on the box 17 by two resilient elements, forinstance pneumatic pressure cushions 24C and 24B (which extendtransversely through the machine). The cushion pressures can be adjustedindividually. The front pressure cushion 24C could be replaced by ajoint the axis of which extends transversely through the machine. Abovethe curved suction box 16 there is a secondary headbox 10'. Above theshoe 9B and the ledges 27 there is again a top wire 12 which travelsover wire guide rolls 14 and 19 and over rigid ledges 28' and 28 of anupper water-removal box which is otherwise not shown. The front wireguide roll 14 is located at only a slight distance from the wire supportsurface 9. Here, the twin-wire zone begins; it ends at a separationsuction box 23A. The twin-wire zone extends initially with slightinclination downward and then with slight inclination upward to the saidseparation suction box 23A. The rigid ledges 28 are adapted to thiscourse of the twin-wire zone; the same is true of the resilient ledges27 supporting the bottom wire and of the shoe 9B. Its length L (in thedirection of travel of the wire) is about 40% of the length of thesandwich zone S.

The twin-wire former shown in FIG. 3 again has a substantiallyhorizontally extending but slightly upward curved twin-wire zone. Itcomprises three sections, I, II and III, arranged one behind the other.The endless wire belts (bottom belt 11 and top belt 12) which are shownonly in part, travel in the immediate vicinity of a headbox 10 overseparate breast rolls 13 and 14, respectively, so that the two wirebelts form a wedge-shaped entrance gap 15 at the start of the twin-wirezone. The jet of pulp given off from the headbox 10 comes into contactwith the two wire belts 11 and 12 first of all at the place where thebottom wire 11 travels in the first section I of the twin-wire zone overa stationary curved forming shoe 16. The curved travel surface of thelatter is formed of several ledges 16' (with water-removal slots presentbetween them) and of an adjoining shoe C which forms a water-impermeablewire support surface 9. The distance between the two breast rolls 13 and14 is variable. The forming shoe 16 can be operated with or withoutvacuum. It can be supported rigidly or resiliently (for instance, bymeans of pneumatic pressure cushions) on a machine frame, not shown (orby means of a joint on the feed-side end and by means of a pressurecushion only in the region of the shoe 9C).

In the second section II of the twin-wire zone, the two wire belts 11and 12 (with the fiber suspension which is in part still liquid presentbetween them) travel between a lower water-removal box 17 and an upperwater-removal box 18. In the lower water-removal box 17 there are aplurality of ledges 27 (of approximately rectangular cross section)which, as in FIGS. 1 and 2, are pressed resiliently from below againstthe bottom wire 11.

The upper water-removal box 18, which is developed as shown in FIG. 1,has a plurality of rigid ledges 28 on its bottom side. In the region ofthe forming shoe 16, part of the water of the fiber suspension isdischarged downward; another part penetrates-- due to the tension of thetop wire 12-- upward through the top wire and is deflected by thefrontmost ledge of the ledges 28 into the front vacuum chamber 21. Thewater penetrating upward between the upper ledges 28 passes into therear vacuum chamber 22. The water penetrating through the lower wire 11between the lower ledges 27 is discharged downward.

In the third section, III, of the twin-wire zone, both wire belts 12 and13 travel over another curved forming shoe 23 which (as shown) isarranged preferably in the lower wire loop 11. Following that, anadditional ledge 29 with vacuum chamber 30 can be provided in the loopof the top wire 12. Furthermore, flat suction boxes 31 can be providedwithin the loop of the bottom wire. There (as shown by dash-dot lines)the top wire 12 can be separated by means of a guide roll 19 from thebottom wire 11 and from the fiber web formed. The bottom wire and thefiber web then travel over a wire suction roll 20. The guide roll 19can, however, also lie further towards the rear, so that the top wire isseparated from the bottom wire 11 only at the wire suction roll 20.

The distance between the two wires 11 and 12 has been exaggerated in thedrawing. In this way, it is intended to make it clear that the two wires11 and 12 converge towards each other over a relatively long path withinthe twin-wire zone. This indicates that the process of the formation ofthe web commences relatively slowly on the first forming shoe 16 (insection I) and is completed only in section III. In this connection, theend of the main water-removal zone in which the two wires convergetowards each other (and thus the end of the web-forming process) lie,for instance, approximately in the center of the wrapping zone of thesecond forming shoe 23, as shown, for example, in FIG. 3. The end of thewire convergence is indicated symbolically there by the point E; at thatpoint the solids content of the paper web has reached a value of about8%. This point can, however, also lie, for instance, on one of the flatsuction boxes 31 or in the end region of section II.

The embodiments shown in FIGS. 4 and 5 differ from the others primarilyby the fact that the twin-wire zone rises substantially vertical fromthe bottom to the top. In this way, the discharge of the water removedfrom the fiber suspension is simplified, since the water can bedischarged substantially uniformly towards both sides. In particular, novacuum chambers are required in the middle section II of the twin-wirezone. The forming shoes 16, 23, particularly those arranged in the thirdsection III, can, if necessary, be provided with a suction device.

Further elements of the twin-wire former shown in FIG. 4 are a formingsuction roll 40 as well as various water-collection containers 41, 42and 43 and furthermore guide plates 44 which are associated with thestationary ledges 28, as well as a water-discharge ledge 45. The otherelements are provided with the same reference numerals as thecorresponding elements in FIG. 3. The same applies to FIG. 5. Withregard to further details of the embodiments according to FIGS. 3 to 5,reference is had to Patent Application PCT/EP 90/01313 which isequivalent to ± WO 91/02842.

In FIG. 4-- similar to FIG. 2-- a shoe 9D having a substantiallywater-impermeable surface is provided at the feed end of thewater-removal box 17, and therefore in front of the resilient ledges 27.In FIG. 5, on the other hand, such a shoe 9E is arranged in front of therigid ledges 28.

The embodiments in accordance with FIGS. 3 to 5 have the feature incommon that each of the shoes 9C, 9D and/or 9E temporarily brakes theremoval of water through one of the two wires. This increases (asalready explained) the quality of the web. Furthermore, the possibilityis obtained of controlling the distribution of the fines and fillersover the thickness of the web (by varying the position and/or the lengthof the substantially water-impermeable wire support surface 9).

FIG. 6 differs in only a few details from FIG. 1: The lowerwater-removal box 17 now has two rigid ledges 8 below the wire guideroll 14. A substantially water-impermeable plate 9F is substantiallyshorter in the direction of the travel of the wire than in FIG. 1; itslength L is only about 20% of the length of the sandwich zone S. It liesbelow the first three upper ledges 28', 28 and therefore exclusivelywithin the sandwich zone, and is supported on the rigid plate 26 byledges 27B and pneumatic pressure cushions 24B. As an alternative toFIG. 6, the following is possible: Each of the ledges 27B has a widenedhead over which the bottom wire 11 slides. In such case, the plate 9Fwould be eliminated.

FIG. 7 shows further possible modifications of the embodiment shown inFIG. 1: The two water-removal boxes 17 and 18 form a sandwich zone Swhich is slightly inclined downward (with respect to the direction oftravel of the wire). The wire guide roll 14' is developed as formingroll (i.e. with water-storage properties in the roll jacket) and isarranged at a shorter distance from the first upper ledge 28', so thatthe water which is slung off by the roll 14'passes into the front vacuumchamber 21. The substantially water-impermeable plate 9G which isresiliently supported on the rigid plate 26 rests at its feed end in apivot joint 2 and at its discharge end on two pneumatic pressurecushions 24B (or on one of them). The initial region of the plate 9G iscurved in order to deflect the bottom wire 11 which arrives inhorizontal direction into the inclined sandwich zone S. Somewhat beforethe curved region a secondary headbox 10' is arranged, so that the jetof pulp 1' emerging from it impinges in the curved region on the (inpart still liquid) fiber suspension 1 arriving with the bottom wire 11.The upper water-removal box has an extension in the form of a curvedsuction box 23' which again deflects the two wire belts 11, 12 upwardand effects a forced removal of water from the web formed. The featuresof FIG. 7 described above can be used individually or in combinationwith each other in the twin-wire former of FIG. 1. The wire guide roll14' which is developed as forming roll and brings the top wire 12 intodirect contact with the fiber suspension can assure an earlycommencement of the removal of water through the upper wire and possiblya certain flattening of the jet coming from the secondary headbox 10',if said jet is to be somewhat undulated over the width of the wire.

FIG. 8 shows possible modifications of FIG. 2. Instead of thewater-impermeable shoe 9B (FIG. 2), a perforated plate 9H is provided aspart of a suction box 17' which is supported rigidly (or resiliently) onthe rigid plate 26. The plate 9H forms a wire support surface 9" whichis of only limited water permeability so that, in its region, theremoval of water in downward direction is braked but not completelyprevented. In general, the following applies: The wire support surface9" can be provided with continuous holes or slits. It is alsoconceivable for the plate 9H to have grooves or furrows on its surface.The slits, grooves or furrows can extend parallel to the direction oftravel of the web or form an acute angle with it, which angle ispreferably less than 45°.

Upon the manufacture of the said plate 9H, one can now so select thepercentage of the open surface, referred to the entire surface of thewire support surface 9", in such a manner that the water permeability ofthe wire support surface assumes as precisely as possible the valuewhich results in the desired improvement in the quality of the finishedfiber web. As a rule, the open surface will be made relatively small sothat the water permeability of the wire support surface 9" issubstantially less than the water permeability of the lower wire 11. Avacuum which is variable during operation can be maintained in thesuction box 17'. In this way it is possible to control, within widelimits, the speed of the removal of water which takes place through thebottom wire 11 in the region of the wire support surface 9" duringoperation. If the speed of water removal is to be kept relatively low inthe region of the wire support surface 9", the vacuum will be set to avery small value, possibly to a value of zero. As an alternative tothis, one can, if necessary, establish a certain pressure within saidbox. In such case, the wire support surface 9"acts precisely as thoughit were water-impermeable.

For this purpose, a conduit 30, which can be connected by a switch 31(indicated only symbolically) either to a suction blower 32 or to asource of compressed air 33, debouches into the suction box 17'. Thus, avacuum or pressure can be established as desired in the suction box 17',its value being variable by means of a control valve 34.

FIG. 9 shows details of the plate 9A which was only indicated in FIG. 1and of the corresponding pressing device. Two wire belts 11 and 12,namely a bottom wire 11 and a top wire 12, travel in the directionindicated by the arrow R. Only the first two ledges 28' and 28 of anupper water removal box are indicated, they extending transverse to thedirection of travel of the wire. The plate 9A is supported on astationary water-permeable plate 26 by ledges 27A via pneumatic pressurecushions 24A, U-shaped ledges 60 being fastened on said plate. Theseshaped ledges 60, the pressure cushions 24A which lie therein and theledges 27A, as well as the plate 9A, extend transverse to the directionof travel R of the wire over the entire width of the machine. By varyingthe pressure in the pressure cushions 24A, the plate 9A can be pressedby the ledges 27A with a selectable force against the bottom of thebottom wire 11. If necessary, the plate 9A can also be lowered downwardfrom the bottom wire 11. For the vertical guidance of the ledges 27Athere are provided, in accordance with DE 40 19 884 (=U.S. Pat No.5,078,835), individual guide arms 57, 58, arranged in pairs which aredistributed at relatively large distances apart over the length of theledges 27A.

One of the ledges 27A (which are also referred to as "pressing ledges")extends with its head into a transverse groove 53 which is provided onthe bottom of the plate 9A and at the same time forms a bending joint.The feed-side edge of the transverse groove 53 forms a stop 56. It comesagainst the head of the ledge 27 and thus prevents further displacementof the plate 9A in the direction of travel R of the wire. Such adisplacement could be caused by the frictional force of the bottom wire11 on the plate 9A. In the embodiment shown, three ledges 27A areprovided for the supporting of the plate 9A. Differing from this, onlytwo ledges or more than three ledges, could be provided. On the middleledge 27A there is also provided a transverse groove 53' which forms abending joint. In other words, at the place where the heads of theledges 27A rest against the plate 9A, the normal thickness D of theplate is reduced to the value d, for instance to approximately one halfof the normal plate thickness D. The plate 9A in this way has a bendingjoint at each place where the head rests against a ledge 27A. It is thusmade possible that the wire support surface 9 is not exactly flat in allconditions of operation. Accordingly, the travel path of the bottom wire11 also need not be precisely flat in all conditions of operation. Inorder words, the zones of the plate which lie one behind the other (withrespect to the direction of travel of the wire) can be pressed withdifferent forces against the bottom wire. The bendability of the plate9A can be increased at the places where the ledges 27A rest against itby narrow grooves 54; these grooves 54 are worked into the plate fromthe side of the wire support surface 9. Additional transverse grooves 55or 56 (of any cross-sectional shape) can be worked from below into theplate 9A in order further to reduce its flexural stiffness in thedirection of travel R of the wire.

When the two wire belts 11 and 12 travel in approximately horizontaldirection, as shown in FIG. 9, the plate 9A then rests under its ownweight on the ledges 27A. The plate 9A is preferably made of plastic, sothat its weight per square meter of surface is only about 30 kg or less.Therefore, the plate 9A, after it has been lowered, can be removed fromthe machine transverse to the direction of travel R of the wire (andtherefore perpendicular to the plane of the drawing) and be insertedagain in the same or a similar position. If the wire belts 11 and 12 donot extend horizontally, but obliquely or vertically (from the bottom tothe top or from the top to the bottom), it may be advisable to couplethe plate 9A by at least one tension spring 59 to the stationary plate26. In this way, the plate 9A always remains in reliable contact withthe ledges 27A, although no firm attachment is present between theseparts.

A supporting of the plate 9A with the least possible friction by meansof one of the pressing ledges 27A on the stationary structure 57, 26 canalso be achieved in the following manner: A tension spring 71 extends inthe direction of travel R of the wire from the stationary structure 57,26 to a bracket 72 fastened on the bottom of the plate 9A. The tensileforce of the spring 71 thus counteracts the frictional force which thebottom wire 11 exerts on the plate 9A. The amount of the tensile forcecan be adjusted by means of a nut 73, so that it can be adaptedrelatively precisely to the frictional force. Only one tension spring 71is visible in FIG. 1; actually, several tension springs 71, arrangeddistributed over the width of the machine, will be present.

In the embodiment in accordance with FIG. 10, the following is providedin order to secure the plate 9J in the direction of travel R of thewire: From the plate, a projection 56 extends downward and rests againsta roller 52. This roller is rotatably mounted on a bracket 51, which isfastened to the stationary structure. In this way, there is obtained asliding with little friction of the ledges 27A between the guide arms57, 58 upon the placing of the plate 9J against the bottom wire 11. Asan alternative to this, the low-friction supporting of the plate couldalso be obtained by means of a strap 50 one end of which is pivoted tothe plate and its other end to the stationary structure. The plate 9Jcan be formed of a relatively thick but flexible foil, for instancehaving several incorporated layers of reinforcement threads, or-- asshown-- having an incorporated fabric 66.

Further alternatives for the low-friction supporting of the plate 9K areshown in FIG. 11: On one of the ledges 27A, rollers 47 and 48 areprovided on the guide arms 57A and 58A, respectively. In a variant,shown in dot-dash line, the horizontal supporting of the plate 9K iseffected not via the ledges 27A but via at least one additional supportmember 67. The latter is inserted by means of a T-shaped head into aT-groove of the plate 9K and guided in tiltable manner therein; it isfurthermore guided between two rollers 68 and 69 which are mounted onthe stationary structure. It is understood that in this case theprojection 70 on the plate 9K is dispensed with.

In accordance with FIG. 12, the plate 9L is developed as a relativelythin flexible foil. It extends from a first winding device 63transversely through the machine to a second winding device 64. By meansof these winding devices 63, 64, the foil 9L is held under a certaintension; it is furthermore-- as in the other embodiments-- pressed bymeans of ledges 27A resiliently against the lower wire belt 11. Thedirection of travel of the wire in FIG. 12 is perpendicular to the planeof the drawing.

In all figures the resiliently supported ledges 27 and/or 27A are shownas ledges which are independent of each other. Differing from this, twoor more adjacent ledges 27 and/or 27A could be coupled to each other,for instance by means of struts or straps which extend approximatelyparallel to the direction of travel of the wire from ledge to ledge, asshown diagrammatically, for instance, in FIG. 10 at 71.

We claim:
 1. A twin-wire former for the production of a fiber web from afiber suspension, the former comprising:a first and a second papermachine wire belt moving in a web travel direction and forming atwin-wire zone for fiber suspension to move between the belts; a primaryheadbox for delivery of a fiber suspension to the twin wire zone; withinthe twin-wire zone, at the outer side of the first wire belt which isthe side away from the second wire belt, a first plurality of rigidledges arranged at a distance apart from each other, and a water removalbox supporting the first ledges; within the twin-wire zone, a secondplurality of ledges which lie opposite the first rigid ledges and whichare at the outer side of the second wire belt which is the side awayfrom the first wire belt, resilient support means supporting the secondledges to be pressed with selectable force against the second wire belt;and the twin-wire zone having a region of commencement upstream withrespect to the travel direction, an essentially water-impermeable wiresupport surface at the outer side of at least one of the wire belts atthe region of commencement of the twin-wire zone, said wire supportsurface bridging at least two of said respective ledges at the at leastone wire belt, and structured to prevent removal of water from the fiberweb.
 2. The twin-wire former of claim 1, wherein the wire supportsurface is at the outer side of the second wire belt.
 3. The twin-wireformer of claim 1, wherein the wire support surface is at the outer sideof the first wire belt.
 4. The twin-wire former of claim 1, wherein theregion in the travel direction where the first and second pluralities ofledges lie opposite each other, together with at least a part of thewire support surface, form a sandwich zone; the length of the wiresupport surface is in the range of 10% to 60% of the length of thesandwich zone.
 5. The twin-wire former of claim 4, wherein the wiresupport surface lies partly upstream with respect to the traveldirection of the sandwich zone and partly within the sandwich zone. 6.The twin-wire former of claim 4, wherein the wire support surface liescompletely within the sandwich zone.
 7. The twin-wire former of claim 4,wherein the position of the wire support surface is adjustable parallelto the direction of travel of the wire.
 8. The twin-wire former of claim1, wherein the position of the wire support surface is adjustableparallel to the direction of travel of the wire.
 9. The twin-wire formerof claim 1, further comprising resilient elements for pressing the wiresupport surface with selectable force against the respective wire beltat the outside of which the wire support surface is located.
 10. Thetwin-wire former of claim 9, wherein the wire support surface has afront edge upstream with respect to the travel direction and a pivotjoint in the twin-wire former for supporting the front edge of the wiresupport surface for pivoting of that surface toward and away from therespective wire belt with respect to the travel direction.
 11. Thetwin-wire former of claim 1, wherein in the twin-wire zone, the wirebelts travel in a predominantly horizontal path along the traveldirection through the twin-wire zone, whereby the first wire belt is thetop wire belt and the second wire belt is the bottom wire belt;theresiliently supported ledges and the resilient support means are belowthe bottom wire belt.
 12. The twin-wire former of claim 11, whereinupstream of the twin-wire zone with respect to the travel direction, thebottom wire belt travels in the travel direction, and the bottom wirebelt has a single-wire water removal path that is upstream with respectto the travel direction from the twin-wire zone; andthe top wire beltcomes into contact with the suspension on the bottom wire beltdownstream in the travel direction from the single wire removal path todefine the start of the twin-wire zone.
 13. The twin-wire former ofclaim 12, further comprising a secondary headbox for delivery of fibersuspension located shortly in front of the start of the twin-wire zonein the travel direction; the wire support surface directly following thesecondary headbox in the travel direction.
 14. The twin-wire former ofclaim 1, wherein the wire support surface is convexly curved withreference to the respective wire belt passing thereover.
 15. Thetwin-wire former of claim 1, wherein the wire support surface is rigidlysupported on the twin-wire former.
 16. The twin-wire former of claim 1,wherein the wire support surface is water impermeable.
 17. The twin-wireformer of claim 1, wherein the wire support surface includes openingssized and placed for causing a braked discharge of water.
 18. Thetwin-wire former of claim 17, further comprising a pressurizable box andthe wire support surface being provided on the box, the pressure in thebox being variable between positive and negative values.
 19. Thetwin-wire former of claim 9, wherein the resilient support for the wiresupport surface comprises moveable pressing ledges which extendtransversely to the travel direction, resilient elements on which themoveable pressing ledges are supported, and the twin-wire formerincluding a stationary structure on which the resilient elements areguided.
 20. The twin-wire former of claim 19, wherein the wire supportsurface is a plate which rests loosely at the moveable pressing ledges,the plate having at least one stop by which it can be supported on thestationary structure of the twin-wire former with respect to movement ofthe plate with respect to the travel direction.
 21. The twin-wire formerof claim 20, wherein the stop on the wire support plate cooperates withone of the ledges for controlling the movement of the plate with respectto the travel direction.
 22. The twin-wire former of claim 20, furthercomprising a roller rotatably mounted on the stationary structure andthe stop on the plate being positioned to be supported with respect tomovement with respect to the travel direction by the roller.
 23. Thetwin-wire former of claim 22, wherein the stop comprises a membertiltably mounted on the wire support plate.
 24. The twin-wire former ofclaim 20, wherein the wire support plate is supported for being pushabletransversely to the travel direction of the wire belt.
 25. The twin-wireformer of claim 19, further comprising tension springs supporting thewire support surface which is a plate on the stationary structure, andthe spring force of the tension springs is selected to act againstmovement in the travel direction.
 26. The twin-wire former of claim 19,further comprising a horizontal bending joint which extends transverselyto the travel direction, and the wire support surface is a plate whichis flexuarally soft around the horizontal bending joint.
 27. Thetwin-wire former of claim 26, wherein the bending joint is in the regionof one of the ledges and the plate has a groove on the side thereoftoward the bending joint, the groove extending transversely to thetravel direction.
 28. The twin-wire former of claim 9, wherein the wiresupport surface is a plate which comprises a foil having a reinforcementinsert.
 29. The twin-wire former of claim 9, wherein the wire supportsurface is a plate which comprises a foil which is stretchabletransversely to the travel direction.
 30. The twin-wire former of claim29, wherein the foil is held under tension transversely to the traveldirection.
 31. The twin-wire former of claim 30, wherein the foil of thewire support plate is windable and unwindable.
 32. The twin-wire formerof claim 1, further comprising a respective wire support surface at theoutside of both the first and second wire belts.
 33. The twin-wireformer of claim 1, wherein each of the wire belts is a respectiveendless loop wire belt and the respective pluralities of ledges arewithin the loops of the wire belts, and the wire support surface iswithin the loop of the at least one wire belt.
 34. The twin-wire formerof claim 1, wherein the second wire belt is an endless loop wire belt,and the wire support surface is located within the loop of the secondwire belt.
 35. A twin-wire former for the production of a fiber web froma fiber suspension, the former comprising:a first and a second papermachine wire belt moving in a web travel direction and forming atwin-wire zone for fiber suspension to move between the belts; a primaryheadbox for delivery of a fiber suspension to the twin wire zone; withinthe twin-wire zone, at the outer side of the first wire belt which isthe side away from the second wire belt, a first plurality of rigidledges arranged at a distance apart from each other, and a water removalbox supporting the first ledges; within the twin-wire zone, a secondplurality of ledges which lie opposite the first rigid ledges and whichare at the outer side of the second wire belt which is the side awayfrom the first wire belt; resilient support means supporting the secondledges to be pressed with selectable force against the second wire belt;the twin-wire zone having a region of commencement upstream with respectto the travel direction, a wire support surface at the outer side of atleast one of the wire belts at the region of commencement of thetwin-wire zone; the region in the travel direction where the first andsecond pluralities of ledges lie opposite each other, together with atleast a part of the wire support surface, form a sandwich zone, thelength of the wire support surface is in the range of 10% to 60% of thelength of the sandwich zone, the position of the wire support surface isadjustable parallel to the direction of travel of the wire; the wiresupport surface includes a forming shoe having a slide surface thereon,the wire support surface is convexly curved with reference to therespective wire belt passing thereover and is rigidly supported on thetwin-wire former, and the wire support surface is water impermeable. 36.A twin-wire former for the production of a fiber web from a fibersuspension, the former comprising:a first and second paper machine wirebelt moving in a web travel direction and forming a twin-wire zone forfiber suspension to move between the belts; a primary headbox fordelivery of a fiber suspension to the twin wire zone; within thetwin-wire zone, at the outer side of the first wire belt which is theside away from the second wire belt, a first plurality of rigid ledgesarranged at a distance apart from each other, and a water removal boxsupporting the first ledges; within the twin-wire zone, a secondplurality of ledges which lie opposite the first rigid ledges and whichare at the outer side of the second wire belt which is the side awayfrom the first wire belt, resilient support means supporting the secondledges to be pressed with selectable force against the second wire belt;the twin-wire zone having a region of commencement upstream with respectto the travel direction, an essentially water-impermeable wire supportsurface at the outer side of at least one of the wire belts at theregion of commencement of the twin-wire zone; resilient elements forpressing the wire support surface with selectable force against therespective wire belt at the outside of which the wire support surface islocated, the resilient support for the wire support surface havingmoveable pressing ledges which extend transversely to the traveldirection and resilient elements on which the moveable pressing ledgesare supported, the twin-wire former including a stationary structure onwhich the resilient elements are guided; the wire support surface is aplate which rests loosely at the moveable pressing ledges, the platehaving at least one stop by which it can be supported on the stationarystructure of the twin-wire former with respect to movement of the platewith respect to the travel direction; and a roller rotatably mounted onthe stationary structure and the stop on the plate being positioned tobe supported with respect to movement with respect to the traveldirection by the roller.
 37. A twin-wire former for the production of afiber web from a fiber suspension, the former comprising:a first and asecond paper machine wire belt moving in a web travel direction andforming a twin-wire zone for fiber suspension to move between the belts;a primary headbox for delivery of a fiber suspension to the twin wirezone; within the twin-wire zone, at the outer side of the first wirebelt which is the side away from the second wire belt, a first pluralityof rigid ledges arranged at a distance apart from each other, and awater removal box supporting the first ledges; within the twin-wirezone, a second plurality of ledges which lie opposite the first rigidledges and which are at the outer side of the second wire belt which isthe side away from the first wire belt; resilient support meanssupporting the second ledges to be pressed with selectable force againstthe second wire belt; the twin-wire zone having a region of commencementupstream with respect to the travel direction, an essentiallywater-impermeable wire support surface at the outer side of at least oneof the wire belts at the region of commencement of the twin-wire zone;resilient elements for pressing the wire support surface with selectableforce against the respective wire belt at the outside of which the wiresupport surface is located, the resilient support for the wire supportsurface having moveable pressing ledges which extend transversely to thetravel direction and resilient elements on which the moveable pressingledges are supported, the twin-wire former including a stationarystructure on which the resilient elements are guided; the wire supportsurface is a plate which rests loosely at the moveable pressing ledges,the plate having at least one stop by which it can be supported on thestationary structure of the twin-wire former with respect to movement ofthe plate with respect to the travel direction; a roller rotatablymounted on the stationary structure and the stop on the plate beingpositioned to be supported with respect to movement with respect to thetravel direction by the roller; and the stop having a member tiltablymounted on the wire support plate.
 38. A twin-wire former for theproduction of a fiber web from a fiber suspension, the formercomprising:a first and a second paper machine wire belt moving in a webtravel direction and forming a twin-wire zone for fiber suspension tomove between the belts; a primary headbox for delivery of a fibersuspension to the twin wire zone; within the twin-wire zone, at theouter side of the first wire belt which is the side away from the secondwire belt, a first plurality of rigid ledges arranged at a distanceapart from each other, and a water removal box supporting the firstledges; within the twin-wire zone, a second plurality of ledges whichlie opposite the first rigid ledges and which are at the outer side ofthe second wire belt which is the side away from the first wire belt,resilient support means supporting the second ledges to be pressed withselectable force against the second wire belt; the twin-wire zone havinga region of commencement upstream with respect to the travel direction,an essentially water-impermeable wire support surface at the outer sideof at least one of the wire belts at the region of commencement of thetwin-wire zone; resilient elements for pressing the wire support surfacewith selectable force against the respective wire belt at the outside ofwhich the wire support surface is located, resilient support for thewire support surface having moveable pressing ledges which extendtransversely to the travel direction and resilient elements on which themoveable pressing ledges are supported, the twin-wire former including astationary structure on which the resilient elements are guided; andtension springs supporting the wire support surface which is a plate onthe stationary structure, and the spring force of the tension springs isselected to act against movement in the travel direction.
 39. Atwin-wire former for the production of a fiber web from a fibersuspension, the former comprising:a first and a second paper machinewire belt moving in a web travel direction and forming a twin-wire zonefor fiber suspension to move between the belts; a primary headbox fordelivery of a fiber suspension to the twin wire zone; within thetwin-wire zone, at the outer side of the first wire belt which is theside away from the second wire belt, a first plurality of rigid ledgesarranged at a distance apart from each other, and a water removal boxsupporting the first ledges; within the twin-wire zone, a secondplurality of ledges which lie opposite the first rigid ledges and whichare at the outer side of the second wire belt which is the side awayfrom the first wire belt, resilient support means supporting the secondledges to be pressed with selectable force against the second wire belt;the twin-wire zone having a region of commencement upstream with respectto the travel direction, an essentially water-impermeable wire supportsurface at the outer side of at least one of the wire belts at theregion of commencement of the twin-wire zone; resilient elements forpressing the wire support surface with selectable force against therespective wire belt at the outside of which the wire support surface islocated, the resilient support for the wire support surface havingmoveable pressing ledges which extend transversely to the traveldirection and resilient elements on which the moveable pressing ledgesare supported, the twin-wire former including a stationary structure onwhich the resilient elements are guided; and a horizontal bending jointwhich extends transversely to the travel direction, and the wire supportsurface is a plate which is flexurally soft around the horizontalbending joint.
 40. A twin-wire former for the production of a fiber webfrom a fiber suspension, the former comprising:a first and a secondpaper machine wire belt moving in a web travel direction and forming atwin-wire zone for fiber suspension to move between the belts; a primaryheadbox for delivery of a fiber suspension to the twin wire zone; withinthe twin-wire zone, at the outer side of the first wire belt which isthe side away from the second wire belt, a first plurality of rigidledges arranged at a distance apart from each other, and a water removalbox supporting the first ledges; within the twin-wire zone, a secondplurality of ledges which lie opposite the first rigid ledges and whichare at the outer side of the second wire belt which is the side awayfrom the first wire belt, resilient support means supporting the secondledges to be pressed with selectable force against the second wire belt;the twin-wire zone having a region of commencement upstream with respectto the travel direction, an essentially water-impermeable wire supportsurface at the outer side of at least one of the wire belts at theregion of commencement of the twin-wire zone; resilient elements forpressing the wire support surface with selectable force against therespective wire belt at the outside of which the wire support surface islocated, the resilient support for the wire support surface havingmoveable pressing ledges which extend transversely to the traveldirection and resilient elements on which the moveable pressing ledgesare supported, the twin-wire former including a stationary structure onwhich the resilient elements are guided; and a horizontal bending jointwhich extends transversely to the travel direction, and the wire supportsurface is a plate which is flexurally soft around the horizontalbending joint; the bending joint is in the region of one of the ledgesand the plate has a groove on the side thereof toward the bending joint,the groove extending transversely to the travel direction.
 41. Atwin-wire former for the production of a fiber web from a fibersuspension, the former comprising:a first and a second paper machinewire belt moving in a web travel direction and forming a twin-wire zonefor fiber suspension to move between the belts; a primary headbox fordelivery of a fiber suspension to the twin wire zone; within thetwin-wire zone, at the outer side of the first wire belt which is theside away from the second wire belt, a first plurality of rigid ledgesarranged at a distance apart from each other, and a water removal boxsupporting the first ledges; within the twin-wire zone, a secondplurality of ledges which lie opposite the first rigid ledges and whichare at the outer side of the second wire belt which is the side awayfrom the first wire belt; resilient support means supporting the secondledges to be pressed with selectable force against the second wire belt;the twin-wire zone having a region of commencement upstream with respectto the travel direction; an essentially water-impermeable wire supportsurface at the outer side of at least one of the wire belts at theregion of commencement of the twin-wire zone; resilient elements forpressing the wire support surface with selectable force against therespective wire belt at the outside of which the wire support islocated; and the wire support surface is a plate which includes a foilwhich is stretchable transversely to the travel direction, the foil isheld under tension transversely to the travel direction, and is windableand unwindable.